wave-particle duality

The concept in quantum mechanics that energy-carrying
waves can also behave like particles and that
particles can also display a wave aspect. Light,
for example, demonstrates the wave phenomena of diffraction
and interference but, under other circumstances,
appears to be a stream of tiny particles called photons.
Electrons, on the other hand, which normally
behave like particles, can be made to diffract and interfere as if they
consisted of waves.

The double-slit experiment

By the mid-1920s it was obvious, from the photoelectric
effect, from the Compton effect,
and in other ways, that when light interacts
with matter it does so as if it were made of tiny bullets of energy –
photons. The rest of the time, it goes about
as if it were smeared out in the form of a wave. Apparently, light has an
identity crisis, and nowhere was that crisis more evident than in an updated
version of an experiment carried out long before quantum theory came on
the scene.

Thomas Young's double-slit experiment,
dating back to the early 19th century, offers the clearest, most unambiguous
proof of the wavelike personality of light. On the screen at the rear of
the apparatus appears a series of alternating bright and dark bands. Two
waves from a common source, one rippling out from each slit, combine, and
the stripes on the screen speak, unarguably, of the adding and canceling
of wave crests and troughs.

What happens now if we dim the light source? A standard 60-watt light bulb
puts out roughly 150 million trillion photons per second. This vast number
underscores why quantum effects, which expose the discreteness of energy,
go unnoticed at the everyday level: the individual energy transactions involved
are fantastically small. If we want to pursue the question of how light
really behaves, on a tiny scale, we have to turn the lamp in Young's
experiment down – way down. This was first done in 1909 by the English
physicist and engineer Geoffrey Taylor, who was later knighted for his work
on aeronautics. Shortly after his graduation from Cambridge, Taylor set
up a version of Young's experiment using a light source so feeble that it
was equivalent to "a candle burning at a distance slightly exceeding a mile."
Even at this level of illumination the interference pattern showed up. The
dribble of light passing through the apparatus continued to behave in a
wavelike manner.

What if the light source were turned down even further? What if it were
dimmed so much that it effectively spat out single photons? There was no
way to arrange for this to happen in the early 20th century; the technology
needed just wasn't to hand. Fashioning a light source that emits only one
photon at a time isn't as simple as turning on a faucet so that water comes
out drip by drip (after all, each water droplet contains many trillions
of atoms, each of which is more substantial than a photon.) Consequently,
those involved in the formative phase of quantum physics, like those grappling
with early relativity theory, had to rely on gedanken – thought
experiments – to test their ideas. If Young's experiment
could be done using a light source that fired out individual photons, what
would be seen on the screen? The only answer that squared both with experiments
that had been carried out and with the emerging principles of quantum
theory is that the interference pattern would build up, one point at a time.
This ought to happen even if there was no more than a single photon passing
through the apparatus at any given moment. As the English theoretical physicist
Paul Dirac put it: "Each photon then interferes
only with itself."

Today double-slit experiments with single photons are routinely set up as
demonstrations for undergraduates. An arrangement used at Harvard, for example,
employs a helium-neon laser as a light source, two rotatable Polaroid filters
to cut the intensity down to barely visible, and a pinhole that is 26 microns
(millionths of a meter) in diameter at the front end of a PVC pipe. Further
down the pipe is a slide with slits, each 0.04 mm wide, set 0.25 mm apart.
Light from the double-slit then falls onto a sensitive video camera, which
produces an image on a screen in which individual flashes, corresponding
to single photons, can be seen appearing. With the detection equipment in
storage mode, the single flashes of light are captured live, and the characteristic
double-slit interference pattern can be watched building up in real time.
The familiar bright and dark bands, which cry out for a wave interpretation,
emerge like a pointilist painting from the specks that are obviously the
marks of individual colliding particles.

Something very, very strange is going on here. In the single-photon, double-slit
experiment, each photon starts and ends its journey as a particle. Yet in
between it behaves as if it were a wave that had passed through both slits,
because that's the only way to account for the interference pattern that
forms over time. During its flight from source to detector, the photon acts
in a way that defies not only commonsense but all of physics as understood
before Planck and Einstein.

You might say, let's keep closer tabs on each photon in the experiment.
If it's a particle – a single, pointlike entity – it can't really
go through both slits at once, any more than a person can simultaneously
walk through two doorways. What happens if we put a detector on one of the
slits to tell us whether the photon goes through that slit or the other?
This is easy to arrange and, sure enough, the photon is forced to give itself
up. By posting a sentry at one of the slits, we learn which slit each photon
passes through. But in gaining this knowledge, we lose something else: the
interference pattern. Flushed into the open, compelled in midflight to reveal
its whereabouts, the photon abruptly abandons its wavelike behavior and
acts purely and simply like a miniature bullet bound on a straight-line
trajectory. Somehow the existence of the interference pattern is tied to
a lack of knowledge as to which slit the photon actually went through. If
we don't ask where the photon is, it behaves like a wave; if we insist upon
knowing, it behaves like a particle. In classical physics such a situation
would be unthinkable, outrageous. Yet there it is: the act of observing
light makes its wave nature instantly collapse and its particle aspect become
manifest at a specific point in space and time. It's almost as if a photon
knows when it's being watched and alters its behavior accordingly. Evidently,
an enigma lies at the heart of the quantum world that, like a Zen koan,
resists a solution in familiar, everyday terms.